Why I use Julia

Come for the speed. Stay for the productivity.

Dr. Josh Day
4 min readMar 8, 2019

When I am asked why I use Julia, my immediate response is “multiple dispatch”. Julia is well-known for performance, but that is only a part of what keeps me using it every day. Multiple dispatch is a feature where different code is called by a function depending on the types of the arguments. Combined with the JIT (Just-in-time compiler), Julia will automatically compile specialized code for each set of argument types the function is called with.

A simple example

As an example, consider the following simple function:

 f(x) = x + x

I have not given Julia any hint as to what the type of the argument is, but as long as it’s a type that supports addition, Julia will compile an optimized method for it. You can peek at the LLVM compiler code with the @code_llvm macro.

julia> @code_llvm f(1); @ REPL[5]:1 within `f’
define i64 @julia_f_12805(i64) {
; ┌ @ int.jl:53 within `+’
%1 = shl i64 %0, 1
; └
ret i64 %1
julia> @code_llvm f(1.0); @ REPL[5]:1 within `f’
define double @julia_f_12813(double) {
; ┌ @ float.jl:395 within `+’
%1 = fadd double %0, %0
; └
ret double %1

Ignoring some of the details, notice that one method is specific to 64-bit integers and the other for double precision floats!

What about more complicated types?

Here we will use the Distributions package to implement a naive quantile finder using Newton’s Method. Thanks to the folks at repl.it, you can run the example yourself below! It will take a moment to install the Distributions package.

Newton’s method is an iterative procedure that finds the root of a function.

Newton’s Method

For quantiles, we are trying to find the number x, for a given number q (between 0 and 1), such that:

cdf(dist, x) - q = 0

where cdf is the cumulative distribution function for distribution dist. We also need the derivative of the cdf, which is the probability density function, or pdf.

Again, we have not told Julia anything about what the function arguments are, but when you call myquantile with Distributions.Normal(0, 1) and 0.5, Julia will compile specialized code to run the algorithm and then return the median (0.5 quantile) for a standard normal distribution (which is 0).

Notice that myquantileworks out of the box with other distributions! In fact, as long as the function arguments have methods for mean, cdf, and pdf, it will just work! If you were to implement this quantile algorithm in R, you would need to rewrite it for each distribution using the dnorm/pnorm family of functions.


The language you use has a tremendous effect on how you approach problems (see linguistic relativity). I have a background in statistics, so naturally R was one of the first languages I learned. I don’t mean to bash R (language wars are boring) as it is a fantastic tool for data analysis, but I often find myself asking “how do I solve this without a for loop?” since loops are slow in R. In Julia, I have fewer performance obstacles, so my questions are more along the lines of “what are the methods I’m trying to accomplish this task with?”. If I can reduce a task to the operations that need to be performed, it becomes easy to write abstract yet performant code that works with whatever types I throw at it.

Multiple dispatch has become invaluable to how I code, and with Julia you get it along with stellar performance. If you want to see how I use multiple dispatch to get a lot done with very little code, check out my package OnlineStats.jl for calculating statistics/models on data streams with single-pass algorithms.

I hope you try Julia for yourself and have the same experience I had.

Come for the speed. Stay for the productivity.

About the Author

Josh has a PhD in Statistics from NC State University and works as a Senior Research Scientist at Julia Computing. He operates the Julia For Data Science newsletter and maintains many open source packages, available on GitHub.



Dr. Josh Day

PhD Statistician. Musician. Ultimate frisbee addict. Julialang enthusiast. Twitter: @joshday_stats. GitHub: https://github.com/joshday.